Tuned radio frequency receiver

A direct detection receiver is a relatively simple device for receiving radio waves, for example, a radio, as it was mainly used in the early days of this technology and today., Only in very specific situations In contrast to the heterodyne receiver later developed actions frequency selection, high-frequency amplification and demodulation on the same frequency are held. Reaches the receivable frequency for " straight ", without mixing to the demodulator.

Depending on the number of resonant circuits they were called also single circuit, two-or three - Kreiser. Also a name, for example in the form of 0 -V-1 was common. The 0 indicates the number of RF stages, the V of the rectifier stage (valve) and the 1 for the number NF levels.

History

Straight receiver for long-, medium - and short- wave were up in the 1950s, very common.

Examples of direct detection receiver are the detector receiver and the audio receiver. The famous straight receiver in Germany were the People's Receiver. The simple Pendelaudions beginning of the 1950s in the simple FM devices were also straight receiver.

Today it is widely used radio controlled watches, which receive the DCF77; because this frequency of 77.5 kHz only work with a fixed ( very low ), the expense of a superheterodyne receiver would be too great and would bring no benefit.

By establishing itself ends FM radio, this concept became less important, because on the one hand high-quality FM radio can be realized with straight- receivers hardly the other hand, the superheterodyne receiver ( " superhet " ) were always cheaper and the advantage of simplicity more and more into the background occurred.

Due to the shutdown of some European medium wave radio stations less radio stations can be received as just 25 years ago with straight receivers.

Technology

The advantage of straight receivers and reason for the application of this technique is the simple structure, at least when only one resonant circuit is present ( the single circuit ). For multiple tuned resonant circuits, there are at fixed frequency receivers no problems, but rather, when a variation of the receiving frequency is desired, because then the construction because of the synchronization problem becomes more complicated. A direct detection receiver can in principle have no side receiving frequencies or birdies, under which a superheterodyne receiver often suffers.

Coming from the antenna signal of the mixture of the desired receiving frequency f is provided with a filter ( tank circuit ) is selected, the high-frequency signal is amplified, filtered again and may directly applied to the demodulator. Demodulation can be carried out in a separate step with a separate diode or in the last RF stage. In tubes is, for example, lattice rectifying ( like most Röhrenaudions ) or anode rectifying possible.

From this description it can be already the biggest disadvantages of the straight - derived recipient:

• If multiple frequency filters are provided, all filters must have exactly the same center frequency and tune with each other in the choice of different receiving frequencies ( synchronism). This has been achieved for example by means of multiple - variable capacitors, but can only be done with limited accuracy.
• Because of the constant-velocity problems, it is almost impossible with several coupled resonant circuits to build a filter with well-defined transmission characteristic (eg band-pass with a steep drop outside the frequency range ).
• The bandwidth of frequency filters, which are based on resonant circuits is determined by the Q factor and the mid-frequency. Especially at high frequencies ( short wavelength region ), it is almost impossible to achieve a sufficiently narrow bandwidth; the recipient then has a too low selectivity. In straight tunable receivers also the bandwidth of the frequency -dependent, but should always be the same at a radio receiver.
• Since the Q-factor of tunable resonant circuits is usually less than 100 and decreases with increasing frequency, straight receiver for FM are illusory, because no satisfactory selectivity can be achieved.
• Since the amplified high-frequency signal has the same frequency as the input signal of the receiver, the output of the high frequency amplifier can be resumed from the antenna and there is a feedback. As for the demodulator, a certain minimum voltage is required, this problem can not be solved by reducing the high-frequency gain, but it is a high cost for the screening and isolation of the amplifier output from the necessary input.
• The usually too low or not existing overall gain does not allow automatic gain control as in the heterodyne receiver.

Alternative

These problems can be overcome by a superheterodyne receiver ( " superhet "). It makes use of the fact that in the mixture of two frequencies more mixed products are formed. The frequencies generated are doing the same modulation as the useful signal. Is now produced by an oscillator, a frequency with a constant distance from the frequency of the receiving channel, it is only needed after the mixer -band filter, which are fixed to said intermediate frequency. The intermediate frequency can be selected so that the desired filter bandwidth at this frequency can be easily implemented.

Today's applications

Heterodyne receiver offer no advantages at low reception frequency, if only a single radio service is received and the bandwidth required may also be achieved with a straight receiver with resonant circuits. Only when a temperature and aging independence is required, takes you back to it. This is especially true for the reception of the time signal DCF77 signal from. Due to its modulation, no bandpass filters are required, the resonant circuits can be obtained as a commercially available quartz filter, whereby a sufficient temperature and aging independence is assured. Also very simple receiver for the remote control of short-range are still built as a straight receiver. Another field of application are wireless door bells, wireless weather stations operating in the 433 MHz range.

An example of a straight receiver is the integrated circuit ZN414 of Ferranti, which was introduced in 1972 and in a 3 -pin TO- 92 provides a complete straight receiver for use on a 1.5 V battery in the long and medium waveband. The simple structure of the external circuit necessary parts of this IC found inter alia in radio hobbyists distribution. Another application in the hobby area is the reception of long waves with the PC sound card, the amplifier of the sound card takes over the role of " high-frequency" amplifier.